1. Field of the Invention
The present invention relates to an apparatus for, and a method of, making a hard carbon film, or a so-called diamond-like carbon film, having various physical properties comparable to those of a diamond.
2. Description of the Prior Art
Since it has been discovered that a diamond could be synthesized by means of a low-pressure vapor deposition process, numerous reports have been made on various methods for the formation of diamond films and the synthesis of diamond-like carbon films, an example of which includes that made by H. Vora and T. J. Moravec (J. Appl. Phys. 52 6151, 1981). Any of those prior art methods comprises the use of a gaseous hydrocarbon such as, for example, CH.sub.4 or C.sub.2 H.sub.6 which is transformed into a plasma so that radicals and/or ions contained in the plasma can be utilized to synthesize a diamond film or a diamond-like carbon film. In recent years, various improved systems and apparatuses have been suggested whereby a diamond film or diamond-like carbon film can be fabricated by various devices.
The inventors have suggested in, for example, U.S. Pat. No. 4,645,977, a plasma injection CVD (chemical vapor deposition) method effective to synthesize the diamond-like carbon film on a film-like substrate continuously at a high speed. The plasma injection CVD apparatus used to carry out this method is schematically illustrated in FIG. 9. As shown therein, the apparatus comprises a plasma tube 25, equipped with a plasma generating means, and a vacuum vessel 27 for accommodating a substrate 26. The plasma tube 25 and the vacuum vessel 27 communicate with each other so that a gaseous medium introduced into the plasma tube can flow into the vacuum vessel 27 through a nozzle 28. The gaseous medium introduced into the plasma tube 25 is transformed into a plasma by means of an RF power applied to an RF coil 30, and the plasma is then blown onto the substrate 26 by the effect of a pressure difference between the plasma tube 25 and the vacuum vessel 27. At this time, ions contained in the plasma are accelerated by the difference in potential between an accelerator electrode 29, installed inside the plasma tube 25, and the substrate 26 to impinge the substrate 26. Thus, the flow of the plasma of the hydrocarbon gas containing the accelerated ions makes it possible to maximize the use of the ions and radicals in forming the diamond-like carbon film at a high speed.
However, the plasma injection CVD method has been found to have the following problem. The speed at which the diamond-like carbon film is synthesized depends on the number of the ions and radicals that reach the substrate. On the other hand, the number of the ions and radicals is based on the pressure of the plasma tube, the type of the gaseous medium used and the electric power invested. Therefore, in order to increase the speed at which the diamond-like carbon film is synthesized, the plasma tube should be evacuated to a pressure as low as possible. In addition, since the plasma injection CVD method makes use of the difference in pressure between the plasma tube and the vacuum vessel to cause the radicals of the plasma to flow this pressure difference should be as large as possible in order to increase the speed at which the diamond-like carbon film is synthesized.
However, since the difference in pressure between the plasma tube and the vacuum vessel is limited by the flow resistance imposed by the nozzle, in order for the pressure difference between the plasma tube and the vacuum vessel to be increased while the pressure inside the plasma tube is maintained low, the flow resistance imposed by the nozzle must be increased. Because of this, the opening of the nozzle must be as small as possible and this is limited due to the incompatibility between the increase in speed at which the diamond-like carbon film is synthesized and a large surface area over which the diamond-like carbon film is to be synthesized.
Also, according to the plasma injection CVD method, in order to accelerate the ions, a mesh-like electrode is installed within the plasma tube and a positive potential is applied to the electrode. At this time, the plasma exhibits a potential substantially equal to that developed at the mesh-like electrode and the plasma ions are accelerated by the effect of a potential difference between the plasma and the substrate. When the plasma is blown from the nozzle onto the substrate, a portion of the plasma diffuses into the vacuum vessel and this may often constitute a cause of an abnormal discharge. The lower the vacuum exhibited by the pressure inside the plasma tube, the greater the amount of the plasma diffused into the plasma tube and, hence, the more often does the abnormal discharge occur. Since the plasma injection CVD method entails blowing the plasma onto the substrate, the uniformity of pressure at a film forming portion is limited, and it has been found that the uniformity of the film thickness was limited to .+-.5% at an area corresponding to the cross-section of the nozzle. Because of this, in applications in which uniformity is an important factor (for example, when providing a protective film on a magnetic tape), a further improvement in uniformity of the film thickness is desired.